Porous materials are frequently used as spargers in aqueous liquid systems, for example, chemical reactors, biological reactors, wastewater treatment systems, fermentation systems, marine-life hatcheries, aqua-culture systems, and the like. The air or other gases introduced into the systems may be for the purpose of aeration of the liquid, or for reaction with the liquids and/or solids present, or may be introduced for mechanical purposes, such as agitation of the liquids or separation of solids from the liquids by flotation means. For reasons of effectiveness and efficiency it is generally desirable that the gases be diffused into the liquids and dispersed as fine bubbles.
Although porous plastics, porous glass, porous metals, porous ceramics, and the like, have been used as diffusion materials for sparging, they lack in certain areas and have not been entirely satisfactory. For example, hydrophobic porous plastic materials have the drawback that they generate large bubbles, typically 3 millimeters in diameter or larger. Hydrophobic porous plastic materials have the advantage, however, in that they resist penetration of water and other aqueous liquids into their pores. If the water penetration resistance of the porous material is greater than the pressure exerted by the liquid head above the sparger, gas flow through the diffusion material can be stopped or interrupted without a problem, and can be reinitiated by simply raising the gas supply pressure above the liquid head back-pressure.
On the other hand, hydrophilic porous materials such as porous glass, metals, or ceramics, have the advantage that they can generate fine bubbles, typically I millimeter diameter or less, however, when the gas flow is stopped or interrupted, water or aqueous liquids are drawn into or through the pores. This can cause problems by flooding the gas delivery system with liquid and, at the very least, requires much higher gas delivery pressure to expel the liquid from the pores and initiate gas flow through the diffusion material. Furthermore, when a higher gas delivery pressure is required to expel the liquid from the pores than is required to maintain gas flow through the pores, an uncontrolled flow surge of gas may take place as the liquid is expelled from the pores thereby causing undesirable or harmful turbulence in the liquid above the sparger.